Rf power meter employing a travelling wave apparatus and a maser cavity



Oct. 29, 1968 AHN ,408,567

w. K. K 3 RF POWER METER EMPLOYING A TRAVELLING WAVE APPARATUS AND A MASER CAVITY Filed Aug. 20, 1964 2 Sheets-Sheet 1 ELECTRIC FIELD INVENTOR. WALTER K4 KAHN KW, u /3:4 (i 8A A TTORNEY Oct. 29, 1968 AHN 3,408,567

W. K. K RF POWER METER EMPLOYING A TRAVELLING WAVE APPARATUS AND A MASER CAVITY Filed Aug. 20, 1964 2 Sheets-Sheet 2 DETECTOR FIG] WAVEGUIDE HYBRID-T RECTANGULAR WAVEGUIDE MASER OSCILLATOR INVENTOR. WALTER K. KAHN A TTORNE Y United States Patent Office 3,408,567 RF POWER METER EMPLOYING WAVE APPARATUS AND A MASER CAVITY Walter K. Kahn, Brooklyn, N.Y., assignor to Polytechnic Institute of Brooklyn, a corporation of New Filed Aug. 20, 1964, Ser. No. 390,927

ABSTRACT OF THE DISCLOSURE A radio frequency in after specification.

A TRAVELLING This invention relates to a radio frequency power meter operating in the microwave produce an indication of power.

The prior art has utilized temperature sensitive elements of the bolometer and thermocouple type to measure microwave power. It is also common in the prior art to measure microwave power through the use of dummy loads.

The prior art devices have generally proved ineffective in their ability to measure microwave power in that bolometers, for instance, are easily burned out by overment under test.

It is an object of this invention to provide an RF power meter that is capable of measuring high microwave power with low loss.

tromagnetic wave.

Another object of this invention is to provide and RF power meter which may be incorporated into the equipment to be tested, and which may be used during normal operation of the equipment. A complete understanding of axial waveguides taken along the line A-A' of FIGURE 1 where the top waveguide is shown as rectangular in cross section;

FIGURE 3 is a view in cross section of a pair of coaxial waveguides taken along the line B-B' of FIGURE 1 7 static electrical field 3,408,567 Patented Oct. 29, 1968 where the outer waveguide is shown as a semicircle in cross section;

FIGURE 4 is a view in cross section of a pair of coaxial waveguides taken along the line C-C' of FIGURE 1 where the outer waveguide is shown as circular in cross section;

FIGURE 5 is a view in cross section of a pair of coaxial waveguides taken along the line DD of FIGURE 1 cavity is shown;

FIGURE 6 is a diagram showing the H mode;

FIGURE 7 is a block diagram of one embodiment of the invention.

In general, the objects of the invention are obtained by causing the beam of gas molecules to interact with the RF content in the traveling wave.

It has been the practice in maser technology to use a having an intensity E to act upon a beam of gas molecules. The gas molecules commonly are of the ammonia type.

is the molecular dipole element.

Since the force experienced by the molecules in the beam does not depend on the sign of the field, then for harmonic electric fields the RMS value 17 5E max Furthermore, the use of a harmonic field permits utilization of the desirable H mode pattern in circular waveguides, and this mode pattern cannot be achieved with a static field.

This principle was utilized in the present device in that since an RF field may be used to focus or defocus a molecular beam then by using the focused or defocused beam in a maser amplifier the amount of amplification obtainable .is in proportion. tothe quantity of .focusing or.

defocusing and thus usable as a measure of RF power.

Referring now to FIGURE 1, the elements essential to the interaction area of this invention are shown as an elongated waveguide having concentric coaxial waveguide sections 11, 12 and 13 contained within the waveguide 10 and a maser cavity 14 contained within the waveguide section 13.

The waveguide 10 provides a means by which an RF wave may be introduced into the coaxial waveguides and withdrawn from them.

In one embodiment of this invention, the elongated waveguide 10 is rectangular in cross section at both extreme ends of the waveguide and cylindrical in cross section throughout the middle portion of the waveguide. The waveguide 10 constitutes a rectangular to circular waveguide transducer at its left end and a circular to rectangular waveguide transducer at its right end.

i The transition from a rectangular waveguide to a circular waveguide is given in FIGURES 2, 3, and 4. In FIGURE 2, the rectangular portion 10a of the waveguide 10 is shown as a cross sectional view taken along the line A-A' of FIGURE 1. In FIGURE 3 the semicircular portion 10b of the waveguide 10 is shown as a cross sectional view taken along the line B-B' of FIGURE 1. In FIGURE 4 the full mechanical transition to a circular structure 10 is shown as a cross sectional View taken along the line C-C of FIGURE 1. When the gap in the concentric circular waveguide 11 and 12 is designed in conformity with well known directional coupler art, power is conducted through the gap from the left outer coaxial region into the right inner circular waveguide region. Thus, the circular waveguide invention constitutes a square TE electric rectangular to TE round transducer on the left and a circular electrical TE round to TE rectangular transducer on the right. Thus, the field configuration in the rectangular guide on the left is forced into the desired configuration in the cylindrical guide in the center by shaping the walls from the rectangular cross sectional configuration to the cylindrical cross sectional configuration with the reverse process being followed on the right-hand side of the elongated waveguide 10.

, The cylindrical waveguide sections 11, 12, and 13 are contained within and coaxial with the elongated waveguide 10 and are so designed in diameter and so placed within the waveguide 10 that substantially all of the RF power entering the waveguide 10 at the entrance 15 will be transferred through the coaxial waveguide 10 of outer diameter D into the cylindrical waveguide 12 of diameter In the preferred embodiment of this invention, the traveling wave will set up an H mode pattern within the circular waveguide. The diameters D and D are in the ratio (2) D X 3.832

where X is nth positive root of the first order function, i.e. the nth root of:

Bessel The distance L is chosen so that the traveling wave entering at 15 will be transferred to the concentric coaxial waveguide section 12 through the distance 16 separating the concentric coaxial waveguide section 11 from section 12. The required L is approximately given by:

where B is equal to the propagation constant of the T E iuode (H mode) in the circular waveguide of diameter D and B is equal to the propagation constant of the 10 in one embodiment of this TE mode (H mode). in thecircular waveguide of diameter D Through the same choice of constants, the traveling wave will exit from the waveguide section 12 in the distance 17 separating the waveguide sections 12 and 13. The RF wavewill: thenexit from'the elongated waveguide through the rectangular exit 18. a

In the preferred embodiment of the invention, RF power wascaused-to enter the elongated waveguide 10 through microwave permeable windows into 15. In a like manner RF power was caused to exit from 18. Thus, the interaction region of the RF power meter 'ca'n'be connected into existing operating equipment for the purpose of obtaining a power measurement while the operating equipment is in use.

The pattern associated withI-I mode as shown in FIGURES is not possible with static fields and' thus can only be produced with the RF field as used in the present invention. The electric field intensity of the H mode is zero along the center axis and increases radially to a maximum at some point between the center of the waveguide and the outer wall of the waveguide. The field at the outer wall is again zero. This mode pattern constitutes an ideal pattern for causing the selective population, or population inversion of the molecular gas beam as the gas beam is forced to travel through the mode pattern. The effect of such a pattern on a beam of gas molecules having two of its energy states, an upper energy state and a lower energy state, such as to cause those molecules existing in the upper energy state to move towards regions of low field intensity. Molecules existing in the lower energy state will tend to move towards regions of greater field intensity, cf. Equation 1. With respect to the H mode pattern, this means that the upper energy state molecules will move towards the center of the pattern whereas molecules existing in or having energy states of the lower level will tend to move away from the center of the pattern out to the areas of greatest field intensity. Obviously, the more energy which exists in a traveling wave, the greater the intensity of the field lines of the pattern. Thus, if a beam of gas molecules is subjected to a traveling wave having the H mode pattern, then the molecules of the upper state will exist at the center of the beam and molecules of the lower state will tend to move away from the beam, the amount of molecules or number of molecules moving away from the beam being dependent upon the intensity of the field which in turn is dependent upon the power content of the traveling wave.

In the preferred embodiment of the invention, a beam of ammonia molecules is formed in the nozzle 19 contained concentrically and coaxially within the waveguide section 11, The beam of molecules may be supplied from any conventional source of such molecules. In the preferred embodiment the source is a gas reservoir containing ammonia under pressure. The nozzle 19 may be of any cross sectional configuration, for example, it may have a ribbon configuration, however, in the preferred embodiment of the invention, the nozzle is cylindrical in shape and thus the beam of gas molecules exiting at 20 into the waveguide of 11 will also be cylindrical in shape.

The beam of gas molecules interacts with an RF wave 1n the waveguide section 12, thus causing the beam to be focused or defocused and causing the depopulation of lower energy state molecules from the beam, the quality of focusing being dependent on the energy in the RF field.

The now selectively focused beam of gas molecules impmges on the entrance aperture 21, FIGURE 6, of the maser cavity 14. The defocused molecules, generally consisting of those molecules of the lower state which have been separated out from the beam but are still contained within the waveguide, will enter the waveguide 13 and are collected at the apertures 22, FIGURE 6, surrounding the maser cavity 14. These latter molecules then may be pumped out of the waveguide 13 by any suitable pumping means.

It was found that not all of the RF power enters the l of gas molecules with a traveling wave containing RF supported by the radially exten ing fing r 24 c ri n energy such that said beam and said traveling wave intert e wavegu de section 12 is used to absorb residue power act; to focus molecules from said beam, the amount of Th v q sypqwere fi fii k may be pe m e y ma focusing being dependent on the RF energy content of =P fi j 'lf g 0 1?! f0! PWPQS? iS p in i said traveling wave means operatively associated with said g nera d 1 1 h material s ,1" f. ab rp nfi 10 combining means for detecting the amount of focusing RF n syl A i r achieved by said combining means and for producing an T maser cavlty 14 1s h lll 21 3 qfwa eoutput signal dependent on said amount of focusing; and

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A device for detecting. and indicating the RF power m i t g m t e ma er ca ity T e amplified Slgial in a traveling wave comprising in combination means for is removed through the waveguide section 26 :Itwill be supplymg a beam of gas molecules to an elongated Wave- .obvious to those skilled Qinthe artthat the amount of 53 3:. $223: fg zggg fi 3552;355:5222 253 5 2 31 3; assistan s?unsaturated;mat: 2r beam of means existing in the maser w- I iie flilii i gfiiis ifiiaii 532135152111Ti??? i gfiiiliiifi s. w 5 s iif gg ggg gii f fzs ii i F gg ifg gfii h said source of standard frequency signals to said cavity supplies the beam of molecules to the interaction region means; means for removmg the resul-tant slgnal from sad The molecular beam of gas molecules Interacts Wlth cavity means, means for applying said resultant signal to an RF Wave, the power Inherent m whlch Is to be meas a detector, means for supplying a second signal from said ured and Supplied from a suitable source of such Waves source of standard frequency signals to said detector, and The focused molecular beam is intercepted by a maser lheahs for lhdlcahhghhy uhbalahclhg i h the h cavlty 29 Where the maser is used as an amphfier 20 ginggswiiipplied to said detector as an indication of said A source of signals 30, shown in the referred embod'- ment of this invention as a maser osgillator in that a A device for detechhg. h lhdlcahhg the RF power maser oscillator is a convenient and standard source of m a trhvhhhg wave compnslhg 1h comhlhahoh means for electromagnetic waves of suitable frequency, supplies a shpplylhg a beam of gas molecules having energy h Signal to the Waveguide hybridm The hybridJ. will the energy states associated with which molecules is cause the signals supplied from the masfir oscillator to be dependent on the intensity of electromagnetic fields in such Split {along the Signal paths 32 and 35' That pom-on of manner that the spatial dependence of the fields acts to the Signal traveling along the waveguide path 32 is Passed separate molecules in one class of energy states from those through an attenuator 33 and then into the maser cavity molecules m a second class of energy States to an elon- 29 The amplified signal is removed from the maser cavity gaied wavegmde means for admhhhg an RF f to by a Waveguide 36 which waveguide 36 communicates sa d waveguide; means to cause said RF wave to interact with a Waveguide hYbridqwith said beam of gas molecules 30 that the molecules in The split signal from the Waveguide hybridq, 31 is one of said energy states will be subtracted from the beam also caused to pass through an attenuator 34 along the the number of molecules Shhtracted belhg dependent on Waveguide section 35 to a phase shifter 38. Thus, the sigthe RF power m sa1d travehng Wave means for removing nal propagated along the waveguide 35 is phase shifted gas molecules subtracted i h sa1d beam mhahs for rewith respect to the signal transmitted along the waveguide celvmg the gas beam cohtalhlhg molecules m both energy path 32. The phase shifted signal is then transmitted to states; means for supplylhg. a first standard Slgnal to sad the waveguide hybrid T 37 by a section of Waveguide receiving means so that said standard signal may be am- The attenuators 33 and 34 and a phase shifter 38 may plified in said receiving means, means for removing an take any of several common forms as used in the waveoutput slghal from sa1d recelvmg means and .supplylhg guide art. For instance, the phase shifter 38 could be a Sad output slghal to a deha'ctor; means F h h a Secpinched section of waveguide 0nd standard signal to said detector; and ind cating means The frequency of the maser oscillator 30 and the center fohdlhplaylhg the.reshhaht output from sa1d detector as frequency of the maser cavity 29 are substantially the ahmdlcahoh of sa1d RF P same and are brought to coincidence by adjusting the A h for detectlhg.ahd.lhdlcahhg h RF power maser oscillator and/Or the maser cavity 29 in a traveling wave comprising in combination a means The signals, the amplified signal Carried by the for supplying a beam of NH molecules to an elongated path 36 and the phase shifted unamplified signal transwavegmde means for supplying RF waves to sa1d elonmitted by the path 39 and received by the Waveguide gated waveguide three coaxial Waveguide sections conhYbridJ, 37 are received in a conventional detector 40 tamed within said elongated waveguide in such a manner The attenuators 34 and 33 and the phase shifter 38 are set that said beam of NH3 molecules passes along the axls so that in the absence of an RF field in the interactlon of said coaxial waveguide sections the distance between region 28, the two signals are in such relationship with the first and second and the Second and thud of said each other that y will cancel each other out m the Waveguide sections being such that substantially all of detector 40 and are absorbed by the load inherent in the 5 the RF waves pass through the second Sectlon m an waveguide hybrid-T 37. In the event that the signal on H01 mode so that sa1d h of NH3 molecules Wlh he path 36 and the Signal on path 39 are unbalanced the selectively focused by said RF waves, means for redetector 40 will then detect the degree of unbalance The moving the defocused molecules from said Waveguides actual amount of difference will be directly dependent means to remove the RF Wave from sa1d Waveguldes, a upon the intensity of the H01 mode pattern h t f maser cavity connected to receive the focused beam of discussed with reference to FIGURE 1, which in turn desa1d a molecules, a maser Osclllatol as a Source Of pends entirely on the power content of the RF field Thus, Standard Signals, a first y in the Output of sa1d the degree of unbalancmg is registered as a current on maser oscillator for splitting the signals from said maser mwroammeter 41 d ill glve a dl ect readmg of th oscillator into two components, an attenuator and wavepower content of the traveling wave. 7 guide for transmitting one of said signals to said maser cavity so that said signals so conveyed may be amplified by the focused beam of NH molecules in said maser cavity; waveguide means connected to said cavity to transmit the amplified signal from said maser cavity to a' second hybrid-T; an attenuator and waveguide connected to said first hybrid-T for transmitting the other of said signals from said maser oscillator to a phase shifting means; a waveguide for transmitting the phase shifted signal from said phase shifting means to said second hybrid-T; means connected to said second hybrid- T for combining said phase shifted signal from said phase shifter and said amplified signal from said maser cavity to detect any unbalance between said phase shifted signal and said amplified signal; and means connected to said combining and detecting means to indicate said unbalance between said phase shifted signal and said amplified signal where the quantity of unbalancing gives a direct indication of the RF power content of said RF waves.

5. A method of measuring microwave power comprising the steps of: admitting to an elongated waveguide a beam of gas molecules in each of two types of energy states; simultaneouly admitting to said elongated waveguide a radio wave to interact with said beam of gas'mole'cules-so that the number'of molecules in' one state in said gas beam will be reduced relative to the number of molecules in the other state in said gas beam; admitting the resultant beam'into a maser cavity; applyinga signalfrom a standard frequency source simultaneouslyto said maser cavity and to a phase shifting network so that the signal will react with the molecular beam in said maser cavity, comparing said phase shifted signal withlthe output signal from said maser cavity; and detecting and displaying the resultant signal as an indication ofthe RF power of the radio ,wave admitted to said elongated waveguidel Reference s'Cited UNITED STATES PATENTS RUDOLPH'V. ROLINliC, 'Primary Examiner. E. F. KARLSEN, Assistant Examiner. i I 

